Amorphous solids such as coffee foam, toothpaste, or mayonnaise display a transient creep flow when a stress E is suddenly imposed. The associated strain rate is commonly found to decay in time as gamma_ -t-nu, followed either by arrest or by a sudden fluidization. Various empirical laws have been suggested for the creep exponent nu and fluidization time tau fin experimental and numerical studies. Here, we postulate that plastic flow is governed by the difference between E and the transient yield stress Et(gamma) that characterizes the stability of configurations visited by the system at strain gamma. Assuming the analyticity of Et(gamma) allows us to predict nu and asymptotic behaviors of tau f in terms of properties of stationary flows. We test successfully our predictions using elastoplastic models and published experimental results.